Simulation of an extreme rainfall event over Mumbai using a regional climate model: a case study

An endeavor has been made to utilize the ICTP's regional climate model RegCM for simulating one of the most catastrophic rainfall events recorded in the history of Mumbai, India on 26th July 2005. The recent version of the model, i.e., RegCM4.6 has been used to dynamically downscale this extreme event at 25 km horizontal resolution over the South-Asia Coordinated Regional Climate Downscaling Experiment (SA-CORDEX) domain with initial and lateral boundary conditions from ERA-Interim reanalysis (EIN15). Due to the coarse resolution of the EIN15, the rainfall pattern during the extreme rainfall event that occurred over Mumbai is fairly unviable. However, the implementation of the dynamical downscaling using RegCM4.6 successfully able to capture the extreme events. The results indicate the RegCM4.6 using mixed cumulus parameterization scheme (CPS; where the Emanuel scheme is considered over land and the Grell scheme is forced over ocean (EL_GO) capable of downscaling the heavy rainfall event with higher accuracy compared to forcing data. This highly confined event over Mumbai might be a manifestation of the low-pressure area formed over Orissa and the adjoining regions associated with mid-tropospheric cyclonic (MTC) circulation over the western coastal region. A detailed analysis suggests that the RegCM4.6 is able to reproduce the localized event satisfactorily as far as the spatial and temporal aspects are concerned. There is a significant improvement in the model simulated output closer to the observations in terms of qualitative and quantitative analysis of rainfall and large-scale fields. Furthermore, the RegCM satisfactorily simulates the features such as the convergence at the lower level accompanied with the divergence at the upper level, higher cyclonic vorticity near lower level, and presence of an enormous amount of moisture content at different pressure levels.